1. Field of the Invention
The present invention relates to an optical waveguide device, and more particularly to an optical waveguide device used in an optical communication apparatus or an optical measurement apparatus.
2. Description of Related Art
Generally, optical waveguide devices have come into widespread use in the optical communication field and the optical measurement field. The optical waveguide devices include waveguide type optical modulators in which an optical waveguide and a control electrode are formed on a substrate having an electro-optical effect.
In order to realize a widening of the bandwidth regarding optical modulation frequency, it is important to achieve velocity matching between optical waves and microwaves which are modulated signals and, as such, various methods have been hitherto proposed. For example, the various methods include increasing the thickness of the buffer layer, increasing the aspect ratio of the electrodes, a ridge structure, and the like. Furthermore, Japanese Unexamined Patent Application Publications Nos. S64-18121 and 2003-215519 disclose the following method: The optical waveguide and the control electrode are embedded in an extremely thin substrate having a thickness of 30 μm or less. The thin substrate is bonded to another substrate (a reinforcing plate) having a permittivity lower than that of the substrate. Thus, the effective refractive index of the microwaves decreases and the velocity matching between the microwaves and the optical waves is performed.
As described above, by using the thinned substrate, the degree of freedom in the design of the optical modulator is dramatically increased. For example, even without the use of a buffer layer, it is possible to produce a wideband and low-driving-voltage optical modulator. For example, when the optical waveguide is formed in the optical waveguide device, a metal such as Ti is thermally diffused into the substrate at a high temperature, thereby increasing the refractive index of the diffused portion to be higher than other portions and containing light. Then, the substrate is thinned by the polishing and the optical waveguide device is formed.
However, as disclosed in Japanese Unexamined Patent Application Publication No. 2003-215519, by using a planarized reinforcing plate or an adhesive layer having a thickness of 200 μm or less, the long-term temperature drift and DC drift are improved. However, since the electrodes are directly formed on the substrate, when a voltage is applied to the electrode, an electric field is concentrated on the corner portions of the electrode. In response to this, an electrical charge is locally concentrated on a portion in the substrate which is a dielectric substance. Moreover, a phenomenon occurs in which the driving voltage instantaneously surges (the instantaneous DC drift). The stress caused by the surge may damage the thinned substrate, creating a significant problem.
In order to cope with this problem, as disclosed in Japanese Patent No. 3001027, it is possible to form the buffer layer (0.25 μm to 2.5 μm thickness) including oxide of indium and silicon oxide between the electrode and substrate. However, the process for forming the buffer layer is an additional requirement and the buffer layer has a thickness of 0.25 μm or more. Thus, this causes an increase in the driving voltage.